Trjanic legions were amougn the most rough and tough troops the world has seen ever, period; even in one one one combat, they have a chance agiasnt the japanese samurai, for roman trainign in general was bent on being able to hover between one on one fighting, and formation fighting; I weill remind you all, that, unliek th emass media migth have you belive, the Roman army rarelly used "sheild wall" tactics; hence the reason why each trooper was givien 3 feet between him, and any other fighitng trooper; usch was for the optimal spacieng to allow each trooper to fight proficientlly agiasnt an opponent, while, if needed, beign able to recieve support from his buddies if in a pinch.
I reming you, that samurai use long swoards, and if anything, the Roman army was geared to take on such oppoenets, and the Trajanic era legionaries are the apex of this movemtn, winnign great victories agiasnt the european Kingdom- the Dacians- who used a weapon most similer to hwat wa sin the european arsenal at the time, to the japanese style swoard, and while the dacians were l;ackign in the armour department, it didtn particurlay matter in Roman tactics the amoutn of armour the opponent had; the general aim was to stab at the face, and while this would have been hmpered by the masks samurai wore, the high leval of training, combined with the sixer of the Roman galdius itself help to balence out this fact.
The reason the legions failed, i will remign you all, was NEVER because of a lack of martial profiency, but becaus einternal squables, and economic collpse made the legions too expensive to maintian, thus the empire was forced to look to cheaper alternatives